restoring to mavericks from yosemite, partition disappeared after fail

I have an mbpro 15" with 2 hard disks: Main: SSD with OS Extra: Time machine backups and data

For different reasons I wanted to downgrade to Mavericks. I rebooted with cmd+r and went to time machine backups. Selected from the Extra HD and selected the Main to restore to. Insert Filevault PW, etc, next, etc. I clicked 'restore' and then it gave me an error: "cannot restore" Without any specifics.

Rebooted and tried again but this time I didn't see my Main. I selected the time machine backup, because the Extra hard disk is there. But I couldn't choose Main as a restore point!

I checked with disk utility:

Both HD's are there. But it seems the partition dissapeared on my Main. I can't repartitions or format, the options aren't there.

Verify and repair disk didn't mention anything.

Who can help me restore to Mavericks? If not possible, restoring to yosemite is fine too, as long as I can use my Mac!